In recent years, a technique for forming a thin film transistor (TFT) by using a semiconductor thin film (having a thickness of approximately several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors are applied to a wide range of electronic devices such as ICs or electro-optical devices, and prompt development of thin film transistors that are to be used as switching elements in image display devices, in particular, is being pushed. As a transparent electrode material needed in an image display device such as a liquid crystal display, a metal oxide typified by indium oxide is used.
In addition to a transparent electrode material, there are various kinds of metal oxides, which are used for a wide range of applications. Some metal oxides have semiconductor characteristics. Examples of metal oxides (oxide semiconductors) having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Patent Documents 1 and 2 disclose a thin film transistor in which such an oxide semiconductor is used as a channel formation region and a display device in which the thin film transistor is used for a switching element of a pixel or the like.
Patent Document 3 discloses that the threshold voltage of a thin film transistor including an oxide semiconductor is shifted in the negative direction by ultraviolet light irradiation.